Multiple-input, multiple-output (MIMO) communication systems differ from single-input, single-output (SISO) communication systems in that different data symbols are transmitted simultaneously using multiple antennas. MIMO systems typically employ a cooperating collection of single-dimension transmitters to send a vector symbol of information, which may represent one or more coded or uncoded SISO data symbols. A cooperating collection of single-dimension receivers, constituting a MIMO receiver, then receives one or more copies of this transmitted vector of symbol information. The performance of the entire communication system hinges on the ability of the MIMO receiver to establish reliable estimates of the symbol vector that was transmitted. This includes establishing several parameters, which includes receiver automatic gain control (AGC) for the receive signal.
As a result, training sequences contained in preambles that precede data transmissions are employed to train AGCs to an appropriate level for each receive signal data path. This allows optimal MIMO data decoding to be performed at the MIMO receiver. AGC training and a resulting AGC level typically differ between SISO and MIMO communication systems since the power of the respective receive signals is different. Therefore, a receiver AGC may converge to an inappropriate level for MIMO data decoding if the preamble structure is inappropriate.
For example, a 2×2 MIMO communication system employing orthogonal frequency division multiplexing (OFDM) may transmit two independent and concurrent signals, employing two single-dimension transmitters having separate transmit antennas and two single-dimension receivers having separate receive antennas. Two receive signals Y1(k), Y2(k) on the kth sub-carrier or tone following a Fast Fourier Transformation and assuming negligible inter-symbol interference may be written as:Y1(k)=H11(k)*X1(k)+H12(k)*X2(k)+N1(k)  (1a)Y2(k)=H21(k)*X1(k)+H22(k)*X2(k)+N2(k)  (1b)where X1(k) and X2(k) are two independent signals transmitted on the kth sub-carrier/tone from the first and second transmit antennas, respectively, and N1(k) and N2(k) are noises associated with the two receive signals.
The channel coefficients Hij(k), where i=1,2 and j=1,2, incorporates gain and phase distortion associated with symbols transmitted on the kth sub-carrier/tone from transmit antenna j to receive antenna i. The channel coefficients Hij(k) may also include gain and phase distortions due to signal conditioning stages such as filters and other analog electronics. The receiver is required to provide estimates of the channel coefficients Hij(k) to reliably decode the transmitted signals X1(k) and X2(k).
At the first receive antenna, the time domain channel representations from the first and second transmit antennas are given by h11[n] and h12[n] respectively. A receiver AGC could be trained by employing a single gain training sequence portion of a preamble resulting in a receive signal power of ∥h11∥22 at antenna one of the receiver. Then the AGC level may be derived by employing the receiver analog-to-digital converter dynamic range (ADCDR), the square root of the channel power ∥h11∥2 and a backoff level using the expression ADCDR/(backoff level)/∥h11∥2. The backoff level is a measure of the peak-to-mean receive signal power values expected.
For example, a backoff level of 12 dB (4:1 peak-to-mean) allows for two bits in the ADC conversion to accommodate peak values before clipping occurs. This AGC setting would ensure receiving a maximum signal strength for this backoff level in a SISO system. However, for MIMO operation, both transmit antennas typically emit independent data to give receive signal power of ∥h11∥22+∥h12∥22 at antenna one, for example, which is different than that of the SISO system. This difference can cause clipping of some of the receive signals due to improperly set AGC levels and therefore generate transmission errors. Therefore, a MIMO communication system that provides proper AGC levels for multiple concurrent data transmissions and allows proper AGC levels for SISO or legacy communication systems would be particularly advantageous.
Accordingly, what is needed in the art is an efficient way to provide gain training for MIMO data transmissions that maintains backward compatibility with a legacy communication system.